If a USAA member, a decent discount is available through several providers. Just login the the USAA site and follow the clicks through their member products page.
The airline's can often get you a decent discount with their discount numbers and coupon codes. American Airlines has theirs out in the open for all to see if you wish to grab them and book direct.
For your preferred rental vedor, sign up for their "frequent user program", and add in a credit card # (note: I suggest a one time use or reloadable cc# for this). By being part of their club, you get a few dollars off of the price, and often "head of the line" service.
I used the last two suggestions for an upcoming trip to Kansas City for ScottD's weather seminar. I was able to get through Avis a Chevy Impala (considered a large car) for about $38/day, unlimited miles. And being part of their club, I occasionally get upgrades to bigger cars, and can skip the counter and be dropped right at the vehicle I'm assigned to.